1. Field of the Invention
The present invention relates to a turbocharger having a valve in an inlet of a turbine scroll passage.
2. Description of the Related Art
Turbochargers have been heretofore known in which a turbine shaft with turbine and compressor impellers provided respectively at its two ends is rotatably supported by a bearing housing. A turbocharger of such a type is connected to an engine. An exhaust gas discharged from the engine rotates the turbine impeller, and the rotation of the turbine impeller rotates the compressor impeller via the turbine shaft. Thereby, the turbocharger compresses air as the compressor impeller rotates, and supplies the compressed air to the engine.
Various developments have been carried out for the purpose of enhancing the turbocharging performance of the turbochargers like this. Japanese Patent Application Laid-Open Publication No. 2010-270715 discloses a multi-stage turbocharger in which a low-pressure stage turbocharger and a high-pressure stage turbocharger are provided continuous to each other. Japanese Patent Application Laid-Open Publication No. 2007-192118 discloses a twin scroll-type turbocharger in which two divided turbine scroll passages guide an exhaust gas to a turbine impeller.
When the flow rate of the exhaust gas is low as a result of a low engine speed, the multi-stage turbocharger causes most of the exhaust gas to flow into the low-pressure stage turbocharger after efficiently subjecting the exhaust gas to energy conversion by use of the high-pressure turbocharger with a small capacity. In contrast, when the flow rate of the exhaust gas is high as a result of a high engine speed, the multi-stage turbocharger avoids a rise in the back pressure of the engine by causing most of the exhaust gas to flow directly into the low-pressure stage turbocharger with a large capacity without causing the exhaust gas to pass through the high-pressure stage turbocharger with the small capacity.
The twin scroll-type turbocharger, meanwhile, causes the exhaust gas to flow through one of the two divided turbine scroll passages when the flow rate of the exhaust gas is low as a result of a low engine speed, and causes the exhaust gas to flow through both of the turbine scroll passages when the flow rate of the exhaust gas is high as a result of a high engine speed. Thereby, the response and the rotational torque of the supercharger can be enhanced particularly when the engine speed is low.
For the purpose of switching the passages for the exhaust gas depending on the engine output as described above, each turbocharger includes a valve which is provided in the inlet of each turbine scroll passage. When the valve comes into contact with a corresponding seat surface provided on an exhaust manifold side of the engine, the valve closes a corresponding outlet of the exhaust manifold, and blocks the exhaust gas from flowing into the turbine scroll passage which is provided behind the outlet. Depending on an amount of opening of the valve, the multi-stage turbocharger controls the flow rate of the exhaust gas flowing into the high-pressure stage turbocharger, while the twin scroll-type turbocharger controls the flow rate of the exhaust gas flowing into the one turbine scroll passage and the flow rate of the exhaust gas flowing into the other turbine scroll passage.